1. Field of the Invention
The present invention relates to a Fe-base sintered alloy for valve seats for use in internal combustion engines and more particularly, to an improved Fe-base sintered alloy for valve seats, which possesses a high strength and a high rigidity and hence exhibits an excellent abrasive wear resistance and an excellent corrosion wear resistance, and an excellent lubricity.
2. Description of the Prior Art
There are many kinds of sintered alloys for valve seats for use in internal combustion engines known in the art.
In general, the valve seats can be easily abraded and corroded in internal combustion engines especially, without lubricant and at elevated temperatures. Furthermore, the valve seats can be more easily abraded and corroded in engines which use gasoline containing 1.5 g of Pb per gallon and have multiple valves such as 3-valve, 4-valve, and 6-valve engines.
Thus, when the gasoline containing Pb is utilized in the engines, even though PbO is generated by the combustion of the gasoline functions as a lubricant, if deposits or sludges of Pb compound are disposed excessively around the valve, the heat conduction of the valves through valve seats is obstructed by deposits. Therefore the valve seats can be experienced corrosion wear resistance acceleratly because the valves and the valve seats become over-heated.
Especially for multi-valve engines, even though gasoline does not contain Pb, the valve seats can be readily softened and abraded accelerately at elevated temperature because the valve seats are located nearer one another compared with 2-valve engine.
Thus, generally, the valve seats for internal combustion engines are required to have high wear resistance properties not only at room temperature but also at elevated temperatures, high creep strength properties, and high thermal fatigue strength under repeated impact loadings at elevated temperature.
Such sintered alloy products for valve seats are shown in U.S. Pat. No. 3,982,905 to Osawa et al, U.S. Pat. No. 4,505,988 to Urano et al, U.S. Pat. No. 4,546,737 to Kazuoka et al, U.S. Pat. No. 4,671,491 to Kuroshi et al, U.S. Pat. No. 4,734,968 to Kuroshi et al, U.S. Pat. No. 4,836,848 to Hayama et al, Korean Pat. Publication 89-3408, Korean Pat. Publication 90-6,700, Japanese Pat. Laid Open 56-249, Japanese Pat. Laid Open 56-3654, and Japanese Pat. Laid Open 63-100,206.
However, the conventional valve seats for internal combustion engines generally do not possess desired high strength, high rigidity, excellent abrasion and corrosion resistance properties. Also, the density of several conventional valve seats is too low and hence the seats are too porous and do not provide a valve seat with a satisfactory strength.